In heat exchangers, particularly of the type used to heat or cool liquids such as oil, it is common to use flow augmentation devices to increase mixing or flow turbulence or impede the formation of boundary layers and thus improve the heat transfer efficiency of the heat exchangers. In the past, various types of expanded metal fins or turbulizers have been used. One common type is a corrugated fin where the corrugations are formed with a pattern of slits and the material of the corrugations is displaced laterally to produce offset openings. This produces a serpentine flow path through the turbulizer increasing turbulence and breaking up boundary layers.
Another type of turbulizer is shown in U.S. Pat. No. 4,945,981 issued to Joshi. This patent shows the use of a louvered fin as a turbulizer. Louvered fins are commonly used on the air side of an air to liquid heat exchanger. In this Joshi patent, however, the louvered fin is located inside the heat exchanger tubes or channels that normally contain liquids, such as oils.
Some difficulties with expanded metal or louvered type turbulizers is that they produce undesirably high pressure drops or flow losses in the heat exchanger, or they produce an irregular or non-uniform flow pattern in the heat exchanger passages. This can produce stagnation in some areas of the heat exchanger, but even if this does not occur, a non-uniform flow profile generally indicates less than ideal heat transfer efficiency in the heat exchanger.